powerpc/boot: Add `otheros-too-big.bld` to .gitignore

Currently, `git status` lists the file as untracked by git, so tell git
to ignore it.

Fixes: aa3bc365

 ("powerpc/ps3: Add check for otheros image size")
Signed-off-by: Paul Menzel <pmenzel@molgen.mpg.de>
Acked-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220214065543.198992-1-pmenzel@molgen.mpg.de